I had a red devil, festae, carpintis, jag x flower horn and a random blue African in my 150g up until recently. It was a great mix of cichlids, surprisingly all compatible. Each varying in size from 5 inches up to 14 inches. In my opinion it's always good to have a nice mix so there's always something happening, but that's just me. If I had to choose one to keep on its own, it would have to be the RD, simply because of personality, but they aren't for everyone. Good luck on choosing a good solitary cichlid though mate, and if you can't decide, just buy a group of little tank busters and keep your favorite after they've gained a little size and you know their personalities. It's always fun to watch them grow up